Heat flux tests were conducted with double-walled stainless-steel boiler tubes in a steam generator. The outside of the inner tube was grooved, and the tube was expanded to fit into the inner tube to effect a tight heat transfer bond. Helium in the grooves served as a leak detector system. Heat was transferred at fluxes as high as 450,000 Btu/hr/sq ft from liquid alkali metal on the tube side to water on the shell side at pressure from 113 to 1203 psia. A single walled tube was also tested Methods are presented for calculating the bond resistance. From values of bond and outside resistance. From values of bond and outside resistance a fairly accurate performance estimate may be obtained.